I'm debating between carrying a larger water bottle and a smaller one. A larger one means I'll have to stop less to filter water, but it means I'll be carrying more, at least for a period of time. Even if that temporary added weight isn't an issue, the larger water bottle still takes up more space in my already crowded pack. On the other hand, a smaller water bottle means I'll have to filter more often, but will allow me to to carry less weight and takes up less space.

You have to take into account distance between water sources and weather, among other things. I carry four one-liter bottles, and fill what I'll need plus an extra for a safety margin.
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Don BransonSep 13 '13 at 10:09

8 Answers
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Single use water bottles are nice, like Steed mentioned. I use those a lot. The downside is that most filters don't readily attach to those bottles, which means I often wish I had a third hand when pumping water.

Whenever I have space in my pack, I like to use a hydration bladder. You don't have to take your pack off to drink or ask someone else to hand you your water bottle. There are some filters out there (like the MSR Hyperflow) that connect directly to the drinking hose (you just take the mouth-piece off), so you pump water directly from the source into the bladder in your pack. One other advantage is weight distribution; Most packs have a separate compartment for the hydration bladder close to your back, which is where you want the heaviest items to be. This makes your pack feel more balanced, as opposed to strapping bottles to the outside.

I have a 10 liter bladder that is similar in weight to a nalgene. Most of the time I fill it up with 2 to 3 liters, but if I have to, I can carry water for 3 days in the desert.

The downside to hydration badders is not knowing how much water you have left while you are drinking, and of course the fact that they take up space in your pack.

This depends greatly on where you will be going and therefore how available water is. Dehydration is a serious issue, so if in doubt bring a little extra.

For example, if you're going to be hiking in the Arizona desert in summer, figure you're not going to find any water and you have to bring all that you plan to use. Yes, that could be a lot and it will take some space and have some weight. That's part of the price for hiking in a dry area.

Also keep in mind you'll be using more water than usual in the desert. I can go thru a gallon of diluted Gatorade in 2-3 hours under these conditions. The limiting factor of how far I can hike in this case is how much water I'm willing or able to take.

On the other hand, the White Mountains of New Hampshire are quite wet. You will be frequently crossing or going nearby small streams, sometimes crossing larger ones. Except for some areas above treeline, water is quite available.

However, consider that in many places it's not a good idea to drink the water you find directly. Taking less water in a wet area only makes sense if you can process the possibly dirty and infected water that is available.

I have used filters and boiled water, but recently became aware of a newer technology that blasts the buggers in the water with germicidal UV. See, for example, http://www.steripen.com/ultra. By the way, I wrote the firmware that runs that particular unit, although I am not a employee of Hydro-Photon (the manufacturers of the Steri-Pen line).

Big plastic bottle gives you more flexibility. You can fill it fully or only to the half, or one third.

Two plastic bottles give you even more flexibility. You can take 0,5l water, you can take 1, 2 or 3 liters - depending how much you will need.

0,5 liter water is almost no water. On multi-day trip typical rucksack weights from 20 to 30 kg, so taking 1 liter more water won't make it significantly heavier, while dehydration is an extreme danger. Note that even if there's a spring on the map, or in the place you know, when there wasn't raining for many days, it can be dry, and you'd have to walk many kilometers until you find next one. Doing so with only a small bottle of water simply isn't wise.

@DonBranson, it's quite common for long autonomous trips, when you carry all your food and gear, and don't visit stores (;)) or huts.
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SteedSep 20 '13 at 14:31

@Steed. Okay. Well, my gear for extended trips is about 5.5kg, so that'd be a lot of food to get to 20. Four days' worth would bring me up to about 10.5kg. Many of the long-haul hikers that are doing 3-4 month trips carry about the same weight or less, and re-supply every 4-6 days. Still, if you have to carry food for two weeks, I can see that you could easily get into to 20kg range.
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Don BransonSep 20 '13 at 15:27

Well, my rucksack is ca 3kg, and sleeping bag 2kg. With clothes, food, water, camping equipment and a tent it is hard to go under 20kg
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ВГДЕЖЅZЗИІКЛМНОПҀРСТȢѸФХSep 20 '13 at 21:26

Use common plastic bottles from sparkling beverages. They are lightweight. When they are empty, they can be smashed thin and take almost no space (you can do the same which a half full bottle, pushing out all the air). When you need them again, you inflate them back with water. So you can have a good reserve volume, which costs you nothing when empty.

You can attach them to the exterior of your backpack with straps and they will take no space inside. If you lose one - shame on you for littering, but it can be replaced.

I prefer 1.5 liter ones - as many as you need for you trip and weather.

Your question contains a strawman argument - that you must always fill a large water bottle. Obviously you don't - you can always ½ fill it, and so only have ½ of the weight to carry. Of course, it will still take up the same volume if it's a rigid bottle.

My answer would be to carry two (or more) smaller bottles though. You can fill one or both as you choose, adjusting your load as appropriate to the terrain ahead. The additional benefit of carry two bottles is that you have a backup should something happen to one of them (eg. dropping it down a canyon, it springing a leak and loosing all it's contents, loosing the top, etc). This redundancy can be a lifesaver.

I would highly recommend the 'Platypus' range of flexible water containers - they are incredibly tough, very flexible (will take up essentially no space when empty), lightweight, and yet designed so that it's possible to stand them up. They can also be adapted into a hydration system. Best of all the never seem to taint or go mouldy (unlike Camelbacks). Carry a large one in your bag as your main water reservoir, and a couple of 1 litre bottles for easy access.

No, there isn't any general rule. It depends on a lot of facts, perceptions, and preferences.

I guess the water where you hike is unsafe to drink, and filtering is a reasonable way to make it safe. If you prefer to carry 5 liters of water in order to filter less often, that's what you prefer. If you prefer to carry zero liters of water and filter each time you drink, that's what you prefer.

For people who hike in the Sierra Nevada, backcountry water is safe to drink without treatment [Rockwell 2002], so the answer is different from the answer in your area where the water is unsafe. In the Sierra, where water is generally pretty plentiful, there is seldom any reason to carry any water on your back. Pack an empty Dasani bottle, and when you come to a creek, fill it with nice, safe water, drink it, and then empty the bottle back out before continuing on your hike.

Yet another possibility would be that someone hikes a lot in an area where there's a significant risk of getting sick from viruses in the water. In that situation, filtering won't help, because filtering doesn't eliminate viruses.